One of the most important requirements of a next generation wireless communication system is to support a required high data rate. For this, various techniques such as multiple input multiple output (MIMO), cooperative multiple point transmission (CoMP), relay, etc., have been under research, but the most fundamental and reliable solution is to increase a bandwidth.
However, a frequency resource is in a saturation state at present, and various schemes are partially used in a wide frequency band. For this reason, in order to ensure a broadband bandwidth to satisfy a required higher data rate, a system is designed such that a basic requirement which requires separate bands capable of operating respective independent systems is satisfied, and a carrier aggregation (CA) is introduced. In concept, the CA aggregates a plurality of bands into one system. In this case, a band that can be independently managed is defined as a component carrier (CC).
To support growing transmission capacity, the 3GPP LTE-A or the 802.16m has recently expanded its bandwidth to 20 MHz or higher. In this case, a wideband is supported by aggregating one or more CCs. For example, if one CC corresponds to a bandwidth of 5 MHz, four carriers are aggregated to support a bandwidth of up to 20 MHz.
As such, a plurality of CCs are used in the carrier aggregation system. In this case, in a user equipment (UE), a CC for receiving downlink control information from a base station (BS) may be set differently from a CC for receiving downlink data. Alternatively, links of a CC for receiving downlink control information and a CC for transmitting an uplink signal may follow a link relation different from a link determined in the existing LTE. Such a scheduling method is called cross-carrier scheduling.
In case of cross-carrier scheduling, control information (e.g., downlink control information (DCI)) transmitted by the BS may include a field indicating a specific CC for which the DCI is provided. Such a field is called a carrier indication field (CIF), and a value included in the CIF is called a carrier index.
The DCI transmitted by the BS is transmitted using various DCI formats. Some of the DCI formats are used to provide control information for a downlink to the UE, and other DCI formats are used to provide control information for an uplink.
However, some DCI formats can be used to provide control information for any one of the uplink and the downlink, and can be used for different purposes in the uplink and the downlink. In this case, there is a need to consider a method used by the UE to interpret the DCI transmitted by the BS and a specific component carrier on which the UE performs communication.